1. Field of the Invention
This invention relates to insulated casings for fluid transfer and particularly to a new and improved insulated casing assembly for oil well steam injection which greatly reduces heat transfer between the fluid and the casing components, and permits the outer sections of plural casings to be rigidly coupled together without fluid leakage while the inner sections of the casings are free to expand or contract lengthwise in response to the temperature changes of the fluid which they carry.
2. Description of the Prior Art
Casing assemblies utilized to transfer fluids downhole must be constructed so as to be structurally rigid and leak-proof while being capable of cyclic changes in dimension in response to temperature changes of the fluid flowing through them. This is particularly true when the casing assembly is used to inject very high temperature steam into an oil well. The purpose of steam injection is to lower the viscosity of heavy crude so that it can be pumped or forced to the surface and thus extend recovery. The casing assemblies which are used in such a manner, however, are subject to several potentially destructive forces. Very high static internal and external pressure forces are exerted on the casing walls and the couplings when the assemblies are inserted deep into the ground. Each casing is subjected to the axial force of the weight of the other casings suspended below it in the casing string. The corrosive effects, the erosive effects, and the pressure forces caused by the steam itself on the internal components of the casing as well as the differential thermal expansion of such components caused by the high temperature of the steam and contamination by downhole fluids can cause structural failure of the casing assembly. Insulated assemblies currently used for transporting fluids of less extreme temperatures cannot be readily adapted for oil well steam injection purposes because of the severe conditions encountered downhole in the well. Conventionally insulated flowtubes leave the insulation susceptible to contamination by downhole fluids causing loss of insulating properties and potential failure of the permanent well casing due to overstressing. Another alternate to this method encases the majority of the insulation in a sealed metal jacketing but leaves the joint area completely uninsulated to allow for joint makeup tooling. This uninsulated portion allows high heat transfer locally to the permanent well casing thus producing potential failure stresses in that casing. Previous systems have no provision for accommodating thermal expansion of the flowtube which may amount to more than 10 feet in moderate depth wells and present very difficult sealing problems for the bottom hole packer.
A primary object of the present invention is therefore to provide a new and improved insulated casing assembly for transferring fluids in which an inner fluid-carrying section of each casing is free to incrementally expand or contract in response to temperature changes of the fluid while the outer load bearing section of the casing remains essentially rigid.
Another object of the present invention is to provide an insulated casing assembly in which insulation separating the fluid-carrying portion of each casing from the rigid portions is isolated and thus protected from the fluid.
Another object of the present invention is to provide an insulated casing assembly in which couplings used to join adjacent casings are protected from the high temperature fluid and thus fluid leakage between adjacent casings is substantially avoided.
Another object of the present invention is to provide an insulated casing assembly which insulates along its entire length thus avoiding high heat losses at the coupling area.
Another object of the present invention is to provide an insulated casing with a substantially lower overall thermal conductivity than presently available.
Still another object of the present invention is to provide an insulated casing assembly capable of withstanding static and dynamic high pressure forces without casing assembly failure.